In recent years, split-ring resonators have attracted attention as the resonators of ESR spectrometers. A split-ring resonator is shown in the perspective view of FIG. 1, where a circular cylinder 1 is made from an electrically conductive material. The cylinder 1 is formed with a slit 2, which has a certain width and extends along the central axis 0 of the cylinder 1. A sample (not shown) is inserted into the cylinder 1 along the axis 0. The whole cylinder is shielded by a metallic tube 3. The resonator is connected with an external circuit (not shown) via a loop antenna 4 and a coaxial line 5. This split-ring resonator is higher in sensitivity and permits a larger sample to be inserted into it and examined than the cavity resonator conventionally employed. This split-ring resonator is described in U.S. Pat. No. 4,446,429 and Rev. Sci. Instrum. 52(2), February 1981, pp. 213-216.
The split-ring resonator has some advantages as described above. However, where the specimen to be investigated is so large that it cannot be accommodated in the cylinder, the cylinder must be split to receive the sample. If the specimen is the human body, for example, then such a division of the cylinder is not feasible. In this case, the diameter of the cylinder is large enough to receive a human arm or head, which makes the instrument large and leads to a decrease in the quality factor (Q). Consequently, practical performances such as sensitivity and resolution may not be satisfactorily high.